Fmoc-L-selenomethionine is a protected derivative of the non-proteinogenic amino acid L-selenomethionine. It features a 9-fluorenylmethoxycarbonyl (Fmoc) group protecting the α-amino group, making it a crucial building block for solid-phase peptide synthesis (SPPS). The key structural difference from the natural methionine is the substitution of sulfur with selenium in the side chain. Its primary application is in the synthesis of selenomethionine-containing peptides and proteins. Incorporating selenomethionine into peptides is a standard technique for facilitating structure determination via X-ray crystallography through multi-wavelength anomalous dispersion (MAD) or single-wavelength anomalous dispersion (SAD) phasing. It is also used in biochemical studies to investigate the role of selenium in biological systems, antioxidant properties, and as a tool in redox biology. As an Fmoc-protected amino acid, it is a specialized reagent for peptide chemists and structural biologists. It must be handled under inert atmosphere and stored at low temperatures to prevent oxidation of the selenium moiety. Its use is almost exclusively confined to research and development laboratories focused on peptide/protein engineering and structural analysis.